In the prior art, there is known an automatic planting device for a hydroponic seedling, which includes a seedbed transfer conveyor that transfers a seedbed by performing intermittent rotation and lateral reciprocation, a planting panel transfer conveyor that is provided below the seedbed transfer conveyor and transfers a planting panel by performing the intermittent rotation, several transfer opening/closing holders that are positioned above the front end part of the seedbed transfer conveyor, hold and separate several seedbed pieces positioned collectively in the front end row of the seedbed on the seedbed transfer conveyor, and transfer the seedbed pieces forward, several insertion opening/closing holders that are positioned in front of the several transfer opening/closing holders, hold and lower the seedbed pieces transferred by the transfer opening/closing holders, and plant the seedbed pieces by inserting same into holes of the planting panel on the planting panel transfer conveyor, and an operation mechanism that is coupled to the seedbed transfer conveyor and causes the seedbed transfer conveyor to intermittently reciprocate in a lateral direction by the width of the seedbed piece of the seedbed (see, e.g., Patent Literature 1).
In the automatic planting device described in Patent Literature 1, the opening/closing holder (the transfer opening/closing holder and the insertion opening/closing holder) configured to hold the seedbed using a pair of holding pieces capable of linearly moving closer to and away from each other is used as a seedbed holding unit that holds the seedbed.